Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues pose challenges for traditional nonvolatile memory technology. This has led to investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory, sometimes referred to as resistive random access memory (ReRAM).
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
The resistive switching memory elements generally have a metal-insulator-metal (MIM) structure. In particular, these resistive switching memory elements typically include a metal oxide resistive switching layer between two conductive electrodes. The metal oxide resistive switching layer typically includes a metal oxide layer. Exemplary metal oxide layer materials include HfOx, ZrOx, AlOx, TiOx, TaOx, and the like. Alternatively, the metal oxide resistive switching layer may be a film stack including a metal oxide film, which serves as the host switching material with another metal oxide as the coupling layer. For example, the host switching material may be HfOx, and the coupling layer may be ZrOx, AlOx or TiOx.
A top electrode is typically formed above the metal oxide resistive switching layer(s). The top electrode is usually comprised of a metal nitride layer such as titanium nitride (TiN). Additionally, a current limiting layer may be provided. The current limiting layer may be comprised of a metal nitride, such as hafnium nitride (HfN).
Thus, to fabricate a ReRAM device, both oxide and nitride layers must be formed. In prior art systems, oxides and nitrides are formed in separate, distinct tools. Typically, all of the oxide layers are formed in one tool, such as a cluster tool. Wafers are transferred to a separate, distinct tool for formation of all of the nitride layers. Such processing is slow, and results in low throughput and low yield. Moreover, multiple tools are required and thus the capital and operating costs are high. Further, since the wafers must be transferred between tools, the vacuum environment is broken, and the wafers are exposed to particulate contamination. Accordingly, new developments and processes are needed.